Flame angelfish

Before i start with whats wrong i know the fish need bigger tank... Bought it last wk off some guy and working on building a 55.. Now to the angelfish hes been fine for the last wk but since this afternoon he been now moving at all and stays this way please tell me what to do as one im completely new to this and 2 dont have any qt tank and dont wanna loose him

ph swings or ammonia spikes & low 02 are the first 3 I'd check! They all lead to the stressed swimming surface gasping sorta behaviour. You can't really test 02 but slapping in increased aeration as a precautionary move is safe and will help if it's an ammonia issue as well .